RESUMEN
OBJECTIVE: To determine the accuracy of the Fitbit Flex (FF) activity monitor for assessing preschoolers' physical activity (PA) and sedentary behavior (SED) in free-living conditions. STUDY DESIGN: This is a cross-sectional study of 27 preschoolers (41% girls; age, 4.9 years) who each wore the FF and ActiGraph GT3X+ (AG) simultaneously for 24 hours. Raw data from each device were temporally matched and summarized into estimates of SED, moderate-to-vigorous PA, and total PA after removing sleep and nonwear time periods. Equivalence testing was performed to compare the 90% CIs of the estimates from the FF with the respective equivalence zone (±15% of the mean estimates) from the AG. RESULTS: The FF yielded equivalent estimates of SED (mean difference, 42-71 minutes/day) and total PA (mean difference, 42-70 minutes/day), but lower moderate-to-vigorous PA (mean difference, 29-67 minutes/day) than the AG, on average. The FF showed strong correlations and relatively low mean absolute percent errors (MAPEs) in relation to the AG for SED (r = 0.87; MAPE = 9.1%) and total PA (r = 0.71; MAPE = 14.3%), but a moderate correlation and high MAPE for moderate-to-vigorous PA (r = 0.59; MAPE = 70.1%). CONCLUSIONS: The FF activity monitor accurately estimated the amount of time spent in SED and overall PA in preschool-aged children, but with an underestimation of moderate-to-vigorous PA. Our findings support the use of the FF as a feasible and a relatively accurate PA monitoring tool in clinical and research settings.